The present invention relates generally to safety glass laminates such as those used as automotive windshields and, more particularly, to a method and apparatus for controlling the rotational positioning of a machine used during production to apply pressure to such laminates to remove air from the laminates and to promote bonding between layers of the laminates.
In the manufacture of safety glass laminates, two glass templates, one slightly larger than the other, are initially cut from glass sheets. The templates, separated by a powder coating to prevent bonding, are placed with the smaller template upon the larger template onto a bending jig which is then passed through an oven heated between approximately 1200.degree. F. and 1400.degree. F. to bend the templates to a desired curvature corresponding to the bending jig. After being cooled, the formed templates are separated and sheets of transparent plastic, such as polyvinyl butyral, are sandwiched between the glass templates. The resulting laminates are then heated to around 140.degree. F. and passed through pressure rollers to remove air from and to promote bonding of the glass/plastic laminates.
More specifically, the heated laminates are first passed through a "de-air" machine which comprises a series of pinch roll pairs which are pneumatically or otherwise forced against each other to define a pressure nip through which the laminates are passed. The pinch roll pairs are mounted in a carriage which is rotatable within a support frame about a generally horizontal axis such that the carriage can be rotated as the laminates pass through the nip of the pinch roll pairs. By rotating the carriage as the laminates pass through, the pinch roll pairs can be maintained substantially normal to the laminates which are therefore balanced within the pinch rolls to prevent breakage of the glass templates of the laminates. The de-aired laminates are then heated to somewhere between 160.degree. F. and 180.degree. F. depending on laminate mass, assembly line speed and other variables, by passage through an oven from which the laminates are conveyed to a "tacker" machine which is substantially identical to the de-air machine. From the tacker machine, the laminates are transferred to an autoclave wherein they are subjected to high pressure and temperature to complete the glass/plastic bonding step of their manufacture.
Operation of the de-air and tacker machines have been a continuing source of problems in the manufacture of safety glass laminates. Operating procedures and control systems previously used include manual operation by means of a large control wheel coupled to the carriage supporting the pinch roll pairs. An experienced operator rotates the carriage about a generally horizontal axis to position the carriage such that the pinch roll pairs are substantially normal to the laminates which are therefore balanced within the pinch roll pairs as the laminates pass through the machine. Manual operation requires an operator per machine, makes repeatable performance of the machine effectively impossible due to differing operators and skill levels of those operators, and also results in approximately 10% to 20% breakage of the safety glass laminates. In addition, when the machines are manually controlled, laminates which are broken are not merely cracked such that they can progress cleanly through the machine for later disposal but frequently the glass templates of the laminates are shattered in the machines resulting in stoppage of the assembly line for clean up with resulting lost time and production.
In view of these problems, attempts have been made to automate the operation of the de-air and tacker machines. In one arrangement, the machine is spring biased toward the orientation which is assumed upon exit of a laminate from the machine. The machine is then moved or rotated, manually or hydraulically, in the opposite direction to the orientation required for entrance of a laminate into the machine. Rotational movement toward the spring-biased exit position is then controlled by pulsating application of a motion restraining brake. This system results in a jerky approximation of manual control which does not appreciably improve laminate breakage; however, the manual operators are eliminated except as required for periodic clean up of the machines necessitated by continuing laminate breakage.
In another arrangement, the angular rotational orientation of the carriage of the machine is controlled by a hydraulic cylinder. A series of control switches or a mechanical cam and cam follower are provided to control the hydraulic cylinder in an attempt to simulate and improve on the manual control procedure. Unfortunately, hydraulics tend to produce only a smooth rotational movement as opposed to the more variable manual control and hydraulics are affected by machine temperatures such that substantial variations in operation of the machine occur due to ambient temperature changes. Further, such switch/cam control does not lend itself to quick change over as production changes from one form or design of safety glass laminate to another.
Accordingly, there is a need for a method and apparatus for controlling a de-air/tacker machine used in the manufacture of safety glass laminates and including pinch roll pairs mounted in a carriage which is rotatable about a generally horizontal axis such that the carriage is automatically rotated as laminates pass through a nip defined by the pinch roll pairs to maintain the pinch roll pairs substantially normal to the laminates such that the laminates are balanced within the pinch rolls throughout their passage through the machine to thereby effectively eliminate breakage of the glass templates of the laminates. Preferably, the method and apparatus would permit the automatic rotational movement of the carriage to be selected rapidly to correspond to any one of a number of standard laminate forms or designs which are to be manufactured utilizing the machine and further permit new rotational movement sequences to be defined by an operator of the machine as necessary.